The objective of this research is to investigate the association between genotoxic exposure and the emergence of mutagenic mechanisms involved with the development of primary and secondary malignancies in humans. Our hypotheses are: 1) Genotoxic exposure leading to clonal proliferative bursts will result in the in vivo evolution of genomic instability in nonmalignant T cell clones from subjects with cancer or predisposed to cancer; and 2) T cell clones with genomic instability captured from these subjects are the result of altered gene function/expression in DNA repair pathways. Our specific aims are: 1) to isolate and characterize proliferating T cell clones with genomic instability from subjects with acute lymphocytic leukemia, Hodgkin's disease and chronic ulcerative colitis following therapeutic genotoxic exposure; and 2) to perform four functional assays for DNA repair pathways on T cell mutants with genomic instability to determine DNA repair pathway integrity. T cell clones with genomic instability will be isolated and characterized using the HPRT T cell biomarker system. T cell clonality will be determined by T cell receptor beta gene rearrangement mRNA analysis. Mechanistic studies of clones with genomic instability will initially include functional screening assays for DNA repair pathway defects. These assays include: a) microsatellite instability to test for mismatch repair defects; b) single cell gel electrophoresis and c) a plasmid based luciferase DNA repair assay to test for double strand break repair, nucleotide excision repair, and base excision repair capacity following repair specific genotoxic exposures; and d) chromosome instability by measuring frequency of chromosomal aberrations. Subsequent studies will include microarray gene expression and genotypic analysis of DNA repair defects in clones in which a functional DNA repair defect has been established. This research will provide insight into gene-environment interactions, genomic instability and malignant transformation in humans, In addition, these studies could eventually be translated to the clinical setting by leading to: 1) the direct screening of patients for inherited and acquired genetic defects associated with the initiation and progression of malignant transformation; 2) new therapeutic protocols and drug development that decreases the genotoxicity and the development of proliferative clones with genomic instability as a consequence of antineoplastic therapy; and 3) patient monitoring/screening for genetic mutations that are associated with the development of second malignant neoplasms.